There are several ideas to stop it or at least help. Probably considered drastic at the present time.

Seems to be OK at the moment, the spam registrations are pretty easy to spot when we check the mod control panel, so we have been killing them there and then.

What appears to be happening is a human is registering, then they are entering the account details into whatever bot they are using in my opinion (At least for those spammers that aren't posting straight away). There are a lot of accounts that are banned (for more than a week or so) that are still being used to access the site. My thoughts are if it was still under human control they would've deleted the account by now as dead.

Who knows though, they maybe just thick as poo and not know they are banned.

I think restricting new members to the new members forum (for the first post) is a good idea, it can make legit users feel more welcome as well. At least if anyone replies and says hi... I confess I'm pretty bad at visiting that forum myself

The other weapon could be a (small) delay. A captcha that requires more than one click (you know the kind I mean) and validation email delayed by 15 minutes. This would require more patience on the part of batch registrants, but wouldn't be much of a hindrance to new members.